Terraria map explanation

Take a look at the example output from UberJumper's map viewer. The large, purple, angular structure on the left is the dungeon.

From what I've seen, it is a typical example. Dungeons start at ground level, run more or less vertically down into the rock layer, and then branch out, becoming mazelike. They may extend into the lower rock layer (where some lava starts appearing), but never into the underworld (where the sea of lava is). Dungeons are always underground, but are also always accessible from the surface.

Your spawn point is dead center in the world (horizontally, that is) and the dungeon spawns as far as it can towards either edge of the map (but never both; you only get one dungeon). This means that it is about as far away from the spawn as it is possible to get. Plus, you could always find you've gone the wrong way and be on the opposite side of the map. (If you find an ocean before you find the dungeon, you've gone the wrong way.)

Pretty much all of the biomes (using Terraria's liberal definition of "biome") can be moved to another location, within certain limits.

The most extreme example is a Meteor crash site biome, which can exist anywhere, at any height. At the other end of the spectrum, we have the Dungeon, which cannot be moved at all.

Completely immovable

• The Dungeon, because it requires an unsafe background wall below the Dirt Wall depth (cannot be placed by the player).
• The Underworld, which is restricted to the very bottom of the map
• The Sky, which, like the Underworld, is just there at a certain altitude.

Mostly immovable

• The Ocean is not completely immovable, but very inflexible. It is both restricted to a certain height (above the surface level), and can only exist within 250 blocks from either edge of the map. You can create an artificial Ocean in the middle of the map, but that won't spawn Sharks, nor will it grow Corals. Creating artificial Oceans that sort of float above the real one does work; I have actually done so to farm Corals.